This invention relates to the pelletizing of polymers. In particular it relates to a process for pelletizing thermoplastic polymer fluff or powder. It further relates to the use of nonsolvents to cause the pelletization of thermoplastic polymer fluff.
Fluff, a dry, fine solid, is typically the form which polymer takes when recovered from a polymerization reactor. It is conventionally pelletized through an extrusion process. The fluff is melted and then extruded through a die into strands which are cut into pellets. This process has several disadvantages. The first disadvantage is that the polymer is subjected to nonuniform heating during the processing which causes the polymer to thermally degrade. Another problem is that the presence of air or oxygen can cause the polymer to oxidize. Polymers are also subjected to high mechanical shear in extrusion processes, which causes mechanical degradation of the polymer, especially of high molecular weight polymers such as ultra high molecular weight polyethylene (UHMWPE). Another problem is high energy cost in heating the polymer. The heating process is very energy intensive and practically none of the heat provided to melt the polymer is recovered.
The instant invention solves many of these problems by heating polymer fluff in a liquid, inert medium. The medium provides uniform heating of the polymer. It also provides heating of the polymer in the absence of air or oxygen so that no oxidation is possible. The instant invention also uses no mechanical shear to form the pellets and uses less energy to form the pellets.
An object of this invention is, therefore, to provide a new method of pelletizing polymer fluff.
Another object of this invention is to pelletize polymer fluff without mechanical degradation of the polymer while minimizing or eliminating thermal degradation of the polymer.
Another object of this invention is to provide a means of pelletizing polymer fluff in the absence of air or oxygen.
A particular object of this invention is to provide a more efficient means for pelletizing thermoplastic polymer fluff.
Other objects of this invention will become apparent from the following disclosure.